1. Field of the Invention
This invention relates to videotape recorders and more particularly to a novel and highly effective videotape recorder usable in a high definition television system (HDTV) and configured to record a video signal on slant tracks and record a PCM audio signal on longitudinal tracks.
2. Description of the Prior Art
In a conventional VTR (videotape recorder), tracking servo is provided in order to enable a rotary head properly to scan slant tracks formed on a magnetic tape. As one form of tracking servo, there is known a system that records on a longitudinal track a tracking control signal that is separated from a recorded video signal and has a frequency equal to the frame frequency. Upon reproduction of the recorded tracking control signal (together with the other recorded signals), the system controls the speed of the magnetic tape so as to establish a predetermined phase relationship between a servo reference signal and the reproduced tracking control signal. That is, the control signal is recorded in order to indicate the position where the rotation phase of a head drum and the frame frequency signal separated from the recorded video signal were properly phase-locked at the time of recording.
For example, in a videotape recorder of a one-inch type C format, the tracking control signal is recorded and reproduced by a single stationary control head 35 as shown in FIG. 1. In FIG. 1, reference numeral 34 designates a drum having a rotary head (not shown) mounted thereon and rotatable at 3600 rpm. A magnetic tape TP is wound helically around the circumferential surface of the drum 34 and transported longitudinally.
A predetermined value is fixed for a distance Wp between a head changeover switching point and the control head 35. A discrepancy between the predetermined value and the actual distance Wp does not cause a tracking error in case of self recording and reproduction, i.e., recording and reproduction by the same VTR. Between different VTRs, however, any discrepancy in the distance Wp gives rise to a tracking error. Therefore, in order to ensure interchangeability between different VTRs, they are adjusted upon shipment so that the distance Wp exhibits, as nearly as possible, the standard value in all sets. This is accomplished by reproducing a standard tape on which the position of the control signal is properly recorded. Variation, if any, in the speed of the magnetic tape TP might seem to invite a deviation in the frequency of the control signal recorded on the tape. However, when the tape is reproduced by a different set, the tape speed as originally recorded is restored by the tracking servo. Therefore, if the distance Wp is constant, no tracking error is produced.
Thus, in the arrangement so configured that the control head 35 is used for both recording and reproduction of the control signal, variation in the speed of the magnetic tape does not cause a tracking error. However, for reasons explained in connection with FIGS. 2, 3A-3F, 4, and 5A-5G, in other arrangements using different and separated heads for respectively recording and reproducing the control signal, any variation in the speed of the magnetic tape TP does cause a tracking error, in the absence of the novel timing correction introduced in accordance with the present invention.
FIG. 2 shows a head placement in a videotape recorder using a rotary head to which the present invention may be applied. The magnetic tape TP drawn away from the drum 34 via a guide is transported at a constant speed in the direction indicated by the arrow A by a capstan 38 and a pinch roller 39. Along the tape path are provided in sequence a playback head 21, erasing head 36, recording head 22 and monitor playback head 37. The recording head 22 records the control signal having a field frequency on the longitudinal tracks, and the playback head 21 reproduces the control signal. The distance Wt between the reproducing head 21 and the recording head 22 is fixed by a format to a predetermined value corresponding, for example, to a two-frame period (53.67 mm).
The recording head 22 is a multichannel head for recording the control signal, PCM audio signal, etc., on tracks parallel to the longitudinal direction of the magnetic tape TP. In order to diminish the width of guard bands and enlarge the recording electric current, a thin-film head is used as the recording head 22. The playback head 21 is a multichannel head for reproducing the control signal, PCM audio signal, etc. In the case of the PCM audio signal, different heads are normally used respectively for recording and playback during editing and at other times. The recording head 22 is not, however, used to reproduce the control signal, since the signal-to-noise ratio of a thin-film head renders it unsuitable for use in reproduction; and during editing, the reproduced control signal would be disordered by a disturbance of the recording electric current of the digital audio signal and would render the tracking servo inoperative. Therefore, it is not advisable to use the recording head 22 to effect reproduction.
Both the playback head 21 and the recording head 22 are compound heads: each includes a head for reproducing or recording the control signal and a head for reproducing or recording the PCM audio signal. In order to refer to these heads individually, the PCM audio signal playback head of the playback head 21 is identified as AHP (Audio Head Playback), and the control signal playback head is identified as CHP (Control Head Playback). Similarly, the PCM audio signal recording head of the recording head 22 is identified as AHR (Audio Head Record), and the control signal recording head is identified as CHR (Control Head Record).
FIGS. 3A to 3F show how variation in the speed of the magnetic tape TP in apparatus configured as in FIG. 2 causes a tracking error. FIGS. 3A, 3B and 3C illustrate phases of the control signal during its playback, whereas FIGS. 3D, 3E and 3F illustrate phases of the control signal during its recording.
When the control signal is recorded, phases thereof at a normal tape speed (FIG. 3D), at a fast tape speed (FIG. 3E) and at a slow tape speed (FIG. 3F) coincide at the position of the recording head CHR. However, phase differences are produced at positions separated from the recording head CHR, and these phase differences are a function of the amount of the separation. When recorded, the control signal recorded at the position of the recording head CHR is phase-locked with a predetermined rotation phase of the drum.
During reproduction, since the tracking servo is activated in each case of a normal tape speed (FIG. 3A), a fast tape speed (FIG. 3B) and a slow tape speed (FIG. 3C), phases of the control signal are brought into coincidence at the position of the reproducing head CHP. However, because of variation in the tape speed, phases of the control signal do not coincide at the position of the recording head CHR, and they cannot be controlled into the same phase-locked condition as recorded. For example, when the variation in the tape speed is .+-.0.1%, the phase difference at the position of the recording head CHR is .+-.53.67 .mu.m (+66.7 .mu.s in time). Since the track pitch of the slant video tracks is such that, for example, 16 video tracks are formed in each field period, the above-indicated phase difference of the control signal in case of 16.67 ms/16=1.05 ms causes a tracking error of (66.7 .mu.s/1.05 ms).times.100=6.4%. The tracking error causes a dropout in the level of a reproduced RF signal from the video tracks. Additionally, when the distance Wt between the recording head CHR and the playback head CHP is different from a standard value, there is a possibility that the tracking error will be increased.
As indicated above, the control signal is used for a tracking servo in order to enable the rotary head properly to scan the slant video tacks formed on the magnetic tape. Besides this, the PCM audio signal is recorded in the digital audio tracks parallel to the longitudinal control tracks on which the control signal is recorded.
For recording, reproducing and transmitting a digital signal such as a PCM audio signal, it is essential to predetermine a format including sampling frequency, number of bits per sample, etc.
For example, a business-use PCM audio signal recording process of a stationary head type is disclosed in Japanese laid-open patent publication No. 57-36410 and Japanese laid-open patent publication No. 59-104714, both assigned to the assignee of the present application. This PCM audio signal recording process employs a format of 16 bits per sample, which is acceptable for sampling frequencies of 32 kHz, 44.1 kHz, and 48 kHz, and employs a tape recording pattern including a plurality (8 to 48) of digital audio tracks, two analog audio tracks, a time code track and a control track, all formed on a tape in its longitudinal direction. The audio data is divided into blocks each including a predetermined number of samples (for example, 12 samples), and, a block address being added to each block, the audio data is recorded together with a block synchronization signal, etc., on the digital audio tracks.
In the digital VTR in which a digital video signal is recorded on and reproduced from a magnetic tape, an audio signal is also digitally recorded. For example, a VTR for digitally recording a high definition television signal is disclosed in Japanese laid-open patent publication No. 62-199179, assigned to the assignee of the present application. This prior art shows three examples of audio signal recording modes. However, in consideration of the tape running speed being relatively high (about 805 mm/s), recording of a PCM audio signal using a stationary head is proposed in addition to these modes.
FIG. 4 shows relationships between tracks formed on a tape. Tv indicates a video track formed by a rotary head, TA represents a digital audio track formed in the longitudinal direction of the tape, and T.sub.CTL shows a control track. On the control track T.sub.CTL is recorded a tracking control signal, for example, in field intervals, as shown by vertical lines in the drawing. On the digital audio track TA a PCM audio signal is recorded in sub-blocks together with sub-block addresses at a rate, for example, of 250 sub-blocks per three fields. The phase relationship between the control signal and the PCM audio signal as recorded is properly established on the tape.
Upon VTR reproduction, for purposes of establishing a predetermined phase relationship between an external reference signal and a reproduced control signal, a capstan motor for controlling tape speed is controlled so as to ensure that the video track Tv coincides with the scan orbit of the rotary head. However, tape stretch and variations in mechanical accuracy between different VTRs (manufacturing tolerances) tend to cause a tracking error as indicated by a scan orbit Ts shown by a broken line in FIG. 4. In order to correct the tracking error, the phase of the reproduced control signal is adjusted by a delay circuit. Adjusting the phase of the reproduced control signal is equivalent to slightly shifting the position of the reproducing head in the longitudinal direction of the tape. More specifically, the tracking adjustment implies, in the case of longitudinal tracks, that the position along the tape reproduced in response to the external reference signal is shifted. Therefore, notwithstanding that the tracking adjustment has been effected, upon editing, where a new PCM audio signal is recorded, superimposed on an old PCM audio signal, based on the timing of the reference signal, lines of blocks move by several blocks, causing improper recording. As a result, the interface between the original recorded data and the new data recorded as a continuation thereof is not "seamless." When the spliced portion of tape is reproduced, because of the improperly shifted rows of blocks, several blocks of data are omitted before data playback is restored, and there is a substantial likelihood that uncorrected errors will result.
In order to overcome these problems, Japanese patent application No. 63-93246, assigned to the assignee of the present application, discloses control of the phase of recorded data on the basis of a comparison output which is obtained by comparison of a reference block address signal with a block address of a reproduced digital signal.
The prior proposed system involves the disadvantage, however, that it is not known whether an advance or a delay of a reproduced block relative to a reference block is caused by a tracking adjustment or by an off-phase in a previously recorded PCM audio signal (base recording). Therefore, in case of recording signals in successive superimposition for cut editing, errors between an ideal value and an actual value of the distance between a reproducing head and a recording head of a PCM audio signal are accumulated in successive edits as shown in FIGS. 5A to 5G. The ideal value of the inter-head distance is a value so adjusted as not to disorder the rows of data blocks upon editing in which reproduction is effected without any tracking adjustment and a new PCM audio signal is recorded.
FIG. 5A shows positional relationships of a playback head AHP and recording heads with respect to the digital audio track TA. The recording head shown at AHR.sub.1 is at a proper distance from the playback head AHP. The recording head shown at AHR.sub.2 is less distant than the proper distance, and the recording head shown at AHR.sub.3 is more distant than the proper distance. In case of using the recording head AHR.sub.2 more distant than the proper distance, when new data shown by oblique lines is recorded on a base recording (original data) in continuation with and subsequent to the IN point, the splice becomes discontinuous as shown in FIG. 5B.
When reproducing by means of the magnetic head recorded with signals in this condition and further recording new data on the base which has been spliced once already with a deviation .tau., the earlier-disclosed deviation correcting system produces a deviation 2.tau. twice as great, as shown in FIG. 5C. From here on, in the same fashion, when new data is further spliced with a base which has already been spliced (n-1) times, a deviation .tau. n, i.e. n times the deviation per one splice, is accumulated as shown in FIG. 5D.
In case of using the recording head AHR.sub.2 less distant than the proper distance, when new data shown by oblique lines is recorded on the base in continuation with and subsequent to the IN point, the splice is discontinuous as shown in FIG. 5E. Also in this case, when new data is further spliced with a base which has been spliced once already, a doubled deviation 2.tau. is produced as shown in FIG. 5F, and when new data is spliced with a base which has already been spliced (n-1) times, the deviation .tau.n which is n times the deviation per one splice is accumulated as shown in FIG. 5G.